This invention generally relates to a bone screw for surgically fastening fractured or severed bone fragments. This invention is particularly suitable for use as a hip fixation pin for fractures of the neck of the femur, although is not limited thereto.
The present invention utilizes many of the features of U.S. Pat. No. 4,175,555 to Herbert which discloses a bone screw having screw threads which are like-handed but of different pitch on its respective leading and trailing ends and spaced apart by a smooth, cylindrical shaft. The pitch of the leading threads may exceed that of the trailing threads in order to hold the bone fragments in compressive engagement.
U.S. Pat. No. 3,051,169 to Grath provides a bone screw disclosed as suitable for use with femoral neck fractures which also includes a first leading set of threads and a second trailing set of threads which are indicated to be preferably of the same pitch. The second set of threads is carried on a sleeve member which fits snugly around the unthreaded shaft at its inner end, but then for the remainder of the sleeve's length has a somewhat larger internal diameter to accommodate a helical spring between the sleeve and the shaft of the screw. The sleeve carries a spacing ring to center the sleeve on the shaft. Outside of the ring is another helical spring which is supported against a nut threadably adjustable on the outer threaded end of the screw shaft. The disclosure states that by use of the spring means, the broken bones are forced together to facilitate and expedite healing.
It is also known to use hip screws which include only one set of threads on an elongated shaft. Such a screw is often utilized in a multiple pinning technique, i.e., two or three such screws all being aligned through the head and neck of the femur for fixation of a neck fracture. Examples of such pins are illustrated and disclosed in U.S. Pat. Nos. 3,842,824 and 3,892,232 to Neufeld and U.S. Pat. Nos. 4,383,527 and 4,450,835 to Asnis.
Another type of fixation device commonly utilized for fractures of the femoral neck, is a compression hip screw which typically includes a first angled barrel and plate member and a lag screw member receivable within the barrel. An example of such a compression hip screw is shown in U.S. Pat. No. 4,530,355 to Griggs. Griggs also includes a compression screw which may be inserted through the barrel and threaded into the back portion of the lag screw to obtain a tight compression between the lag screw and the plate/barrel member. Once the desired amount of compression has been achieved, the compression screw may be removed or left in place at the option of the surgeon. In the course of time, absorption takes place near the fractured bone surfaces. Internal forces act on the lag screw/compression screw assembly, causing it to back out of the barrel, and thus protrude beyond the barrel/plate assembly. If the compression screw has been removed, absorption will still cause the lag screw to back out, but it is less likely to protrude as far out of the barrel/plate assembly. Such a compression hip assembly as described above, permits longitudinal sliding movement between the lag screw and the barrel due to the forces produced while such absorption occurs. Griggs also provides a clip which may be optionally insertable into the barrel to prevent axial rotation of the lag screw with respect to the barrel member.
It is noted that while a bone screw such as that described in the previously discussed U.S. Pat. No 4,175,555 to Herbert is very suitable for fractures such as of the scaphoid and other similar small bones, if such a screw were used to achieve compression of a fracture across a femoral neck, absorption could cause the screw to protrude through the femoral head and into the joint capsule which would cause pain to the patient. The device would protrude into the joint capsule as a result of the absorption because as the bone shortens, it is unable to longitudinally slide out the other end as with the compression hip screw of Griggs.